How to set the camera view
A couple strange things to note here...


the Viewport has its own field of view


[N] to match "View" Lens (70.00) to your Camera's Focal Length
It won't be perfect, but it will lessen the difference.


Through the Camera View, you can maximize the look at it with
[Shift]+[Home]


That let's most any view occupy all available space in whatever window the keystrokes apply to.
[Home] (alone) will comfortably fit the view without taking all available space, and


Enable Border in the Dimensions dropdown under the Render Tab


[Ctrl]+[B] left-click drag around the Camera's dotted border to Preview only THAT in Render Mode; it makes things happen faster.

You should be set in your workflow for the most part if I understand what you wanted.